Document EP-A-0 770 514 describes an example of such a hinge mechanism which is capable of remaining unlocked while the seat-back is being tilted down forwards, even after the control member has been released, e.g. to access the back seats of the vehicle if the mechanism in question is mounted on a front seat of a two-door motor vehicle. When the seat-back is subsequently raised, the mechanism automatically locks itself again when the peg comes into register with the corresponding gap in the circular guide.
The mechanism described in that document gives full satisfaction, but it nevertheless suffers from the drawback of always locking itself in the same angular position for the seat-back when it is raised after being tilted down forwards, with this happening regardless of the position into which the seat-back had previously been adjusted.
The user of the seat must therefore readjust the inclination of the seat-back each time it is raised after being tilted down forwards.
Document EP-A-0 694 434 also describes a hinge mechanism, said mechanism having toothed slugs and enabling the seat-back to be tilted down forwards and to lock itself automatically again in its previously-adjusted position. The toothed slugs of that mechanism which serve to adjust the inclination of the seat-back engage in a set of teeth which is itself movable relative to the second cheek-plate and which can be secured to the second cheek-plate by means of other toothed slugs.
Under such circumstances, the two cheek-plates are connected together via two stages of toothed slugs, thereby tending to reduce the mechanical strength of the hinge, or making it necessary to increase the weight and the cost of the hinge mechanism in order to obtain high mechanical strength.